Lena/Nov 2016

November 2016

BACK/NEXT

<< BACK NEXT >>

11/02/2016

Magnetic connectors

It seems like many parts of our magnetometer setup are actually very magnetic. That includes the BNC cables, banana plugs and the circuit boards.

The circuit boards from OSH Park were plated with Ni, and that caused problems.

Replacement non-magnetic connectors:

1. BNC plug http://www.mouser.com/ProductDetail/Amphenol-RF/112710/?qs=sPbJEtTr5b45g4lDF9tMFw%3D%3D
2. BNC jack http://www.mouser.com/ProductDetail/Amphenol-RF/112740-13/?qs=ZnztrWCQEtE9xZQYo%2FWsjg%3D%3D
3. Banana plug http://www.mouser.com/ProductDetail/Pomona-Electronics/3276/?qs=sGAEpiMZZMvdy8WAlGWLcPkjJE6AhYLi
4. Banana jack (except the nut) http://www.mouser.com/ProductDetail/Johnson-Cinch-Connectivity-Solutions/108-0903-001/?qs=sGAEpiMZZMuIC3ROaEqRYUclhTyEUpND
5. Pin headers http://www.mouser.com/ProductDetail/Molex/22-28-4363/?qs=sGAEpiMZZMs%252bGHln7q6pm%252bS0pk2Wo0XxTAhHStsXU8w%3d

11/07/2016

Pump modulation and noise

We are trying to verify whether the drifts in the pump laser are the reason why our measured magnetic field noise is so large. We have really low probe noise, around 1 fT/sqrt(Hz), and the measured magnetic field noise was consistently higher than 10 fT/sqrt(Hz), especially at the frequency below 10-20 Hz in WIMR. The SQUID measurements suggest that it should be ~5 fT/sqrt(Hz).

We modulated the pump current, which modulated both optical frequency and power at 10 Hz. The change in frequency was ~1 GHz, and the change in power was ~0.02%. It produced a 1.5 pT peak in the magnetic signal. That's 1.5 fT per 1 Mhz, or 2 ppm of optical power change. 2 ppm requires at least 19 bits, the feedback loop only has 16. The problem can be circumvented by averaging the AI data and creating a custom PID loop block.

Mike suggested that we measure how the magnetic field depends on the pump optical frequency.

• File:2016.11.07 Pump Modulation Data.pdf

11/08/2016

Stabilizing optical frequency

Built a setup for saturation absorption spectroscopy to stabilize the pump optical frequency in Chamberlain. The current control signal consisted of ~20 MHz sine modulation at 12345 Hz, and a DC feedback signal. The signal was demodulated by a lock-in amplifier and given to a 16-bit digital PID controller. The laser was locked to the brightest visible crossover resonance (~50% contrast). We measured the magnetic field with and without the frequency stabilization, and there was no change in the magnetic noise floor. The magnetic noise remained at 10 fT/sqrt(Hz).

Diff/sum board oscillations

It appears that the output filters on the diff/sum board oscillate at 4 MHz with about 100 mV amplitude. It might be happening because the opamp is directly driving the 820 pT feedback capacitor. We should replace the capacitor with a smaller value and see if that helps, or alternatively we could use a different opamp in the future frontend designs.

11/09/2016

Stabilizing optical power

Added a pickoff made from a microscope slide to the pump path in Chamberlain that picks off pump power and sends it to a photodiode. The photodiode is connected to an SRS 570 current preamplifier with no bias voltage. The pickoff is installed after the fiber output and the polarizer before the cell inside the magnetic shields. The feedback loop is keeping the photodiode current constant by feeding back on the laser current. I attempted to do it with an AOM instead, but it took to long to align it.

The feedback loop managed to stabilize the input signal up to 10^-5, and there were no changes in the observed magnetic field noise. The input reading was most likely incorrect and suffered from the optical interference fringes, because the feedback sign spontaneously changed to the opposite over a course of a few hours.

Mode hopping

We discovered that the DFB pump laser in Chamberlain is mode hopping at the frequency close to the optical transition. That made it difficult to measure how the field response depends on the pump optical frequency. We changed the laser temperature to move the mode hop further away from the transition.

Optical frequency dependence

Measured how the magnetic field change depends on the optical frequency of the pump laser. The pump current is modulated with a sum of two waveforms - 0.1 Hz ramp that sweeps through the optical frequency, and 38 Hz harmonic modulation that changed the optical frequency by 1 GHz. We connected the magnetometer signal to the lock-in to see how both phase and amplitude depend on the optical frequency.

We observed response in both X and Y channels of the lock-in. The X response remained positive, and changed by 20-50%. The Y response was an 10 times smaller than the X response, and changed sign as the optical frequency changed. The pump is parallel to the Z direction and orthogonal to the probe. It appears that the measured magnetic field noise has contributions both from the changes in atomic polarization and light shifts, but the light shifts contributions are an order of magnitude smaller.